Trace amounts of radioactive iodine from Japan’s damaged nuclear reactors have shown up in drinking water in Richland and Boise.

The results are the first discovery by the Environmental Protection Agency (EPA) of radioactive material from Japan in U.S. drinking water. Scientists at the University of California, Berkeley documented tiny amounts in tap water there last week.

The levels of iodine-131 in water samples from Richland and Boise — about 0.2 picocuries per liter — are so small the EPA estimates that even an infant would have to drink nearly 7,000 liters to receive a dose of radiation equivalent to a day’s worth of normal background radiation. Iodine-131 can be harmful in higher amounts, particularly to babies and young children, because it concentrates in the thyroid gland and can lead to cancer later in life.

The EPA’s standard for the maximum level of iodine-131 in drinking water is 3 picocuries per liter.

The EPA says it is not surprising that hints of radioactive material from Japan would find their way into some American water supplies. A faint radioactive plume has spread widely, with trace amounts detected in air and rainwater across much of the U.S.

Also on Monday, the EPA reported elevated levels of radionuclides in rainwater from Olympia, Portland and cities in Alabama, California, Connecticut, Idaho, Minnesota, New York, Ohio and Tennessee. Levels of iodine-131 ranged as high as 242 picocuries per liter in Boise. Levels in Olympia and Portland were 125 picocuries per liter and 87 picocuries per liter, respectively.

Even though those levels are many times higher than the EPA’s drinking-water standard for iodine-131, the agency pointed out that the standard is designed to protect a person who drinks contaminated water daily for 70 years. Because the isotope has a half-life of eight days, levels of iodine-131 are expected to drop quickly.

Rainwater in Boise also contained slight amounts of the longer-lived radionuclides cesium-134 and cesium-137.

A sample of milk from Spokane tested positive last week for what the EPA says are harmless traces of iodine-131.

To determine that the radioactive material came from Japan, experts rely on a combination of timing and elimination of other possible sources. Iodine-131 would not normally be found in Richland’s or Boise’s drinking water and is “consistent with what we expected to see from Japan,” said an EPA spokeswoman.